In the course of receiving telephone calls, subscribers who have more than one local extension, frequently find it necessary to shunt (i.e. put to one side or hold in abeyance), a call received on the incoming line until a desired party is called to the telephone or is transferred from one to another local extension. The shunting of such a call is more commonly known as "holding" of a call. For this purpose telephone instruments are provided with a hold circuit activated by a "hold" key or a "hold" button. By manipulating this key or button a subscriber is able to transfer an incoming call to the holding circuit instead of to a local telephone instrument. This holding circuit is essentially a shunting circuit which simulates the electrical characteristics of the subscriber's local telephone instrument. This allows the handset of the telephone initiating the hold to be replaced "on-hook." The hold will be released when any of the extension telephones on the initiating telephone are taken "off-hook."
Although a hold circuit can be made with only solid-state devices, the complete solid-state circuit requires the use of high current SCR devices, high voltage transistors, and expensive varistors or other devices to protect the circuit from voltage surges. As a consequence, the solid-state circuits are costly and sometimes suffer from poor sensitivity, marginal operation on long loop conditions and an inability to detect high impedance extension phones going "off-hook." Finally, many hold circuits require a separate power source (not telephone line power) usually from the 110 VAC power line creating a possible hazardous condition.